The present invention relates to a fine cell-containing polyester film, and more particularly, to a fine cell-containing polyester film which is excellent in gloss, printability, transport property and/or flexibility; image-receiving papers suitable for a video printer, which are excellent in image-receiving property and transport property, and has a good printability; and release papers suitable for seal print, which are excellent in adhesion to a coating layer applied thereon.
Polyester films have widely used as industrial materials because the films have various well-balanced properties and are excellent in cost performance. Among these polyester films, fine cell-containing polyester films which are produced, as light-weight white opaque films having a cushioning property, by blending polyester with a thermoplastic resin immiscible therewith and molding the resultant blend into a film shape, have been applied, for example, to synthetic papers such as mounts for seal print, base films of magnetic cards or the like. Especially, the fine cell-containing polyester films have been advantageously used as image-receiving papers or mounts (release papers) for products such as seal print or the like, because of an excellent cushioning property thereof.
Recently, the fine cell-containing polyester films have been used as image-receiving papers having the size of post card on which video images are directly printed. In this case, it has been required that opposite surfaces of the image receiving paper have different properties from each other. That is, the front surface of the image-receiving paper on which images are to be printed, is required to have a high gloss (low surface roughness) in order to receive precise print images thereon. On the other hand, the opposite (rear) surface of the image-receiving paper is required to have a low gloss (high surface roughness) in order to impart a high transport property thereto. When the rear surface of the image-receiving paper exhibits a high gloss, there arises a problem that the image-receiving papers are inhibited from being fed each single sheet into a printer (i.e., so-called double transport of the image-receiving papers is caused).
Also, there have been an increasing demand for release papers suitable for seal print. In this case, there arises a problem that the seal print formed integrally from the image-receiving paper and release paper cannot be fed each single sheet to a printer, i.e., the double transport of the seal prints is caused. Further, since the conventional fine cell-containing polyester films are deteriorated in adhesion to a silicon coat, it has been demanded to improve an adhesion property.
The fine cell-containing polyester films have been produced by melt-kneading polyester and a thermoplastic resin immiscible therewith in an extruder, extruding the resultant melted material through an extrusion die on a casting roll to cool and form an amorphous sheet, followed by successively subjecting the obtained sheet to longitudinal stretching treatment, transverse stretching treatment and heat treatment.
In such a process for the production of the fine cell-containing polyester films, the melted material composed of polyester and the immiscible thermoplastic resin is extruded on the casting roll and contacted with a surface thereof. For this reason, a surface of the resultant sheet where the sheet is contacted with the casting roll (cast surface) theoretically exhibits a lower surface roughness than that of the opposite surface of the sheet (air-side surface). As a result, the cast surface of the resultant oriented film shows a larger gloss than that of the air-side surface thereof. Accordingly, in order to eliminate the double transport, it is suitable that the cast surface and air-side surface of the film are used as a printing surface and a back feed surface, respectively.
However, in such a case, since the immiscible thermoplastic resin has a larger affinity to a surface material of the casting roll than that of the polyester, there arises a problem that the immiscible thermoplastic resin is separated from the surface of the polyester sheet when the sheet is removed from the casting roll, thereby depositing on the surface of the casting roll. This results in decrease in gloss of the cast surface of the resultant film, so that the difference in gloss between the cast surface and air-side surface becomes small, thereby failing to provide a printing surface which is required to have a high gloss. Further, when the amount of the immiscible thermoplastic resin deposited on the surface of the casting roll is considerably increased, the deposited immiscible thermoplastic resin is transferred onto a cast surface of respective films subsequently produced, so that uneven patterns tend to be formed thereon, thereby causing severe damage to quality of the products when used for printing.
Furthermore, the gloss of the fine cell-containing polyester film may be increased by laminating a polyethylene terephthalate homopolymer layer containing no fine cells on the fine cell-containing polyester film. However, in this case, in order to impart a desired gloss to the film, it is necessary to increase the thickness of the homopolymer layer. Such an increased thickness of the homopolymer layer causes problems that the flexibility of the film is lost as a whole and the resultant prints are deteriorated in printing density.
As a result of the present inventors' earnest studies, it has been found that by stretching a film made of a polyester composition containing a specific thermoplastic resin in a specific amount in at least one direction, the obtained fine cell-containing polyester film is excellent in gloss, printability, transport property, heat resistance and/or surface smoothness, has an appropriate flexibility and is useful as a base film of image-receiving papers for a video printer and release papers for print seal. The present invention has been attained on the basis of the finding.